Method for preventing reboiler scaling



4-, c. G. GERHOLD 2,365, 3

METHOD FOR PREVENTING REBOILER SCALING Filed Nov. 16. 1942 zzcezlrx r IPatented Dec. 19, 1944..

METHOD FOR PREVENTING REBOILER SCALING Clarence G. Gerhold, Chicago, Illassignor to Universal Oil Products Company, Ch cago, 111., a corporationof Delaware Application November 16, 1942, Serial No. 465,700

3 Claims.

This invention relates to a method for preventing reboiler scaling andmore particularly to a method of operating reboilers which are employedfor heating relatively narrow low boiling range hydrocarbons containingdissolved solids.

The features of my invention make it particularly applicable toisomerization plants in which low boiling hydrocarbons containingdissolved aluminum chloride are heated in reboilers. Difficulty has beenexperienced in these installations due to the deposition of smallquantities of aluminum chloride on the reboiler tubes. This is due tolocalized total vaporization of the liquid surrounding the tubes becauseit is necessary that the tube surfaces in order to supply sufiicientheat for the process be at temperatures higher than the boiling pointsof the various components of the liquid. This local vaporization will,of course, cause a deposit of dissolved solids when the liquidevaporates.

In a short time, the deposition of solids on the reboiler tubes willaffect the heat transfer coefficient and consequntly the efficiency ofthe system. Aluminum chloride besides decreasing the heat efliciency ofthe reboiler also has a corrosive effect on many materials especially ifany 'water is present.

By employing my invention, the above mentioned difficulties due to thedeposition of aluminum chloride on the reboiler tubes can be eliminatedbecause according to my invention I introduce a higher boilinghydrocarbon, which is substantially inert with respect to the aluminumchloride at the conditions of operation, into the reboiler to retain thealuminum chloride always in solution even in the space adjacent thereboiler tubes.

In order to make the features and advantages of my invention moreapparent, reference is made to the accompanying diagrammatic drawingshowing one operation to which it may be advantageously applied.

In the drawing, numeral I denotes a fractionating column which may beany of the conventiona1 types provided with packing or bubble trays andconnected to a suitable reboiler 2 by means of conduits 3 and 4respectively. Reboiler 2 may be any suitable reboiler in which theheating medium such as steam or hot oil is passed in indirect contactand heat exchange relation with liquid from the bottom of fractionatingcolumn I. The particular reboiler illustrated is provided with aplurality of tubular members 5 through which the heating mediumintroduced through line 6 and removed through line 1 is passed but anyother type of heat exchange equipment may be employed, for instance, theheating medium may surround the tubes and the liquid to be heated passtherethrough.

The bottoms product withdrawn through line 8 from the fractionatingcolumn, which in the case wherein this column is used in anisomerization unit for the conversion of normal butane and isobutane,comprises substantially all materials containing four carbon atoms. Thismaterial,

therefore, will have a relatively narrow boiling range which atatmospheric pressure is in the range of about 9 to 32 F.

The overhead fraction from the column is withdrawn through line 9 andcooled in condenser ID by means of a cooling medium supplied thereto bymeans of lines II and I2. The cooled and condensed material is conductedby means of line l3 to receiver 14. In receiver H, the uncondensed gasesare separated from the condensate and removed from the system throughvalve I5 and line IS. The condensate is withdrawn from the receiver bymeans of line I! and pump [8 and all or a portion thereof, returned tothe top of the column by means of valve l9 and line 20 to act as reflux.A portion of this condensate may be removed from the system by means ofvalve 2| and line 22. Feed to the column is supplied by means of line23.

In order to retain the dissolved solids always in solution, a higherboiling material which is substantially inert with respect to thealuminum chloride, at the conditions of operation is introduced throughline 24 and directed either into the feed to the column by means of line25 and valve 26 or it may be directed into the feed to the reboiler bymeans of line 21 and valve 28.

When, for example, fractionating column I is the hydrogen chloridecolumn of an isomerization plant, the feed through line 23 will containa substantial portion of C4 hydrocarbons together with lighterhydrocarbons and hydrogen chloride plus a small portion of dissolvedaluminum chloride. The aluminum chloride will normally remain dissolvedat the temperature and pressure conditions of the column, but when thissolution comes in contact with tubes 5 in reboiler 2 due to thetemperature of the tubes and the relatively narrowlow boiling range ofthe hydrocarbon material, this aluminum chloride would be deposited outas the hydrocarbon vaporizes. When employing my invention, however, thatis, introducing a heavier hydrocarbon material such as alkylate oralkylate bottoms from an alkylation plant, the aluminum chloride willalways be maintained in solution and the difliculties due to scaling onthe reboiler tubes will be eliminated.

I claim as my invention:

1. A method of preventing scaling on tubes in reboilers during theheating of volatile hydrocarhon liquids containing dissolved aluminumchloride which comprises introducing intosaid reboiler a higher boilinghydrocarbon liquid which will retain said aluminum chloride in solutionand which is inert with respect thereto, and maintaining the last-namedliquid at a temperature below the vaporizing point of aluminum chloride.

2. A method of preventing scaling on tubeslin reboilers during theheating of low. boilinghydrocarbons containing dissolved aluminumchloride, which comprises introducing into said ,reb oiler a boiled ahigher boiling hydrocarbon liquid which is relatively inert with respectto and is adapted to retain said aluminum chloride in solution duringthe reboiling, and maintaining the last-named liquid at a temperaturebelow the vaporizing .pointof aluminum chloride.

I CLARENCE G. GERHOID.

